leetcode--First Missing Positive

Given an unsorted integer array, find the first missing positive integer.

For example,
Given [1,2,0] return 3,
and [3,4,-1,1] return 2.

Your algorithm should run in O(n) time and uses constant space.

public class Solution {
    public int firstMissingPositive(int[] A){
        if(A.length==0||A==null)
          return 1;
        //把元素放入正确的位置，例如1放在A[0]，2放在A[1]；负数，相同数字，不必理会，因为已经出现过了,负数则必然连续（但不是顺序的）
        for(int i = 0;i<A.length;i++){
          while(A[i]!=i+1){
            if(A[i]>=A.length||A[i]<=0||A[i]==A[A[i]-1])
              break;
            int temp = A[i];
            A[i] = A[temp-1];
            A[temp-1] = temp;
          }
        }
        
        for(int i = 0;i<A.length;i++){
          if(A[i]!=i+1)
            return i+1;
        }
        return A.length+1;
  }
}